The present invention relates to a step motor drive circuit.
Ordinarily, carriage feed step motors for use in serial printers have three modes of acceleration operation, constant speed operation and deceleration speed operation which are cyclically repeated wherein printing operation is carried out in the mode of constant speed operation.
In general, the modes of acceleration and deceleration operations require a large torque (current) proportional to inertia of load (carriage) as compared to the mode of constant speed operation. Further, it is required for realizing stable start and stop to control the step motor so that the value of a current is kept constant. In addition, even in the mode of the constant speed operation, various kinds of speeds are required in conformity with printing systems or methods employed and it is desirable for providing an optimum printed result to vary a winding current of the step motor in accordance with respective speeds so that its response is substantially equal to the critical response.
In the prior art, drive and control for such step motors has been carried out on the basis of a constant current self-excitation chopper system to detect a winding current of the motor to control an actual winding current by the feedback of the winding current detected, or a separate-excitation chopper system wherein the above-mentioned feedback control is not conducted and a method is instead employed to create a chopper signal to effect a chopper control using the chopper signal.
However, drawback with these conventional systems is as follows. With the self-excitation chopper system, a threshold level for chopping is set in dependence upon a winding current. Accordingly, in the modes of acceleration and deceleration operations and in the mode of constant speed operation, when an attempt is made to vary chopping threshold level to change the winding current, the circuit configuration becomes complicated, with the result that such a system is not practically acceptable. To eliminate this inconvenience, when a control is effected with the chopping threshold level being set at a constant value (e.g. torque at the time of acceleration or deceleration), this torque becomes an excessive drive torque in the mode of the constant speed operation, giving rise to the problem that vibration of the motor occurs, the motor is unnecessarily heated or the like.